"Since the Mass has the same external sign, and the same interior dispositions on the part of Christ, we rightly call it a sacrifice, the continuation of Calvary. It does not need to earn redemption all over - that was done once for all (Hebrews 9:28) by His death. But since the Holiness of God loves everything that is good, and in good order, it pleases Him to have titles or reasons in place for what He will give (cf. Summa I. 19. 5. c). So it pleases Him to have the Mass provide the title for the distribution of what was once for all earned on Calvary."
EWTN claims it is because it pleases God to have a title/reason (excuse/opportunity?) in place for the gifts that God gives ... so the Mass is an opportunity for God to again offer forgiveness purchased "once for all".
At the Last Supper He ordered, "Do this in memory of me". Since we were not there, He wants us to join our dispositions to His.
EWTN claims it is an opportunity for us to obey the command of Christ and join Him at both the Last Supper and His death.
Secondly the people can be said to offer since: "The people join their hearts in praise, petition, expiation and thanksgiving with the prayers or intention of the priest, in fact, of the High Priest Himself, so that in the one and same of offering of the Victim... they may be presented to God the Father "(Acta Apostolicae Sedis, 39:556). Vatican II explains (LG # 10) that this is what it means for them to "offer spiritual sacrifices".
EWTN claims that our "spiritual sacrifices" are offered up to the Father along with the sacrifice of our Lord and Savior.
It may be more "mystical" than I am personally comfortable with, but it does not appear to be claiming "salvation by attending Mass" or "Salvation by eating a cracker" (even a transmutated cracker). Which part is really "heretical"?